F pilin isolated from missense and suppressed amber mutants in the transfer cistrons of an F lac episome is being characterized biochemically in the hope of determining the structural gene for F pilin, and the function of other transfer genes. A series of Flac mutants which are temperature sensitive in replication have also been isolated. These appear to differ from each other in terms of curing rate at the non-permissive temperature. They are being tested for the F properties of incompatibility, and integrative suppression, and we are attempting to devise a means of mapping the mutations employing Hfr deletion mutants. We are also screening for R factor temperature sensitive replication mutants.